System and filter for filtering hard alpha inclusions from reactive metal alloys

ABSTRACT

A system for filtering hard alpha inclusions from a reactive metal alloy, such as titanium, is provided. The system includes a vessel, a receptacle and a filter. The vessel is capable of holding the reactive metal alloy in a molten form, and can pour the molten reactive metal alloy. The receptacle is for receiving the molten reactive metal alloy poured from the vessel. And to prevent at least some hard alpha inclusions from entering the receptacle, the filter is disposed between the vessel and the receptacle such that the molten reactive metal alloy passes therethrough before being received by the receptacle. The filter includes a frame, and a porous surface that is disposed within the frame. The porous surface defines openings that are sized to permit the reactive metal alloy in molten form to pass therethrough while capturing hard alpha inclusions.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under ContractNo. F33657-97-C-0030 awarded by Department of the Air Force. Thegovernment may have certain rights in this invention.

FIELD OF THE INVENTION

[0002] The present invention relates generally to manufacturingcomponents from reactive metal alloys and, more particularly, tofiltering hard alpha inclusions from reactive metal alloys duringcasting of components from the same.

BACKGROUND OF THE INVENTION

[0003] In many industries today, such as the biomedical and aerospaceindustries, components experience severe service conditions and, thus,are often made from titanium alloys or superalloys. For example,turbine-powered aircraft contain critical rotating components in theengine, such as the fan, compressor and the turbine sections, that aremade from titanium alloys and/or superalloys. Generally, such alloys aremanufactured by secondary remelting processes, such as plasma arc coldhearth melting (PAM), electron beam cold hearth melting (EBM), vacuumarc remelting (VAR), and electroslag remelting (ESR). During themanufacturing of the components, quality control takes a significantrole because failure of such components can lead to catastrophic loss ofthe complex system as well as other losses.

[0004] In quality control, one of the most important quality issues fortitanium alloys and superalloys is melt-related inclusions. In thisregard, inclusions can consist of unusually coarse segregated phasesformed in the melt, or as exogenous materials having origins outside thedeliberate alloy constituents. In the case of exogenous materials forinvestment castings, one type of inclusion is mold shell fragments. Moldshell fragments are inadvertently released from the ceramic shell moldduring casting as a result of high thermal stresses and erosion of themold by the molten metal. The ceramic mold innermost layer (that facesthe molten metal) typically contains rare earth metal oxide(s), such aserbia, that are utilized because of their high melting point andchemical compatibility with the reactive titanium melt. Upon release,the mold shell fragments may be incorporated into the body of thecasting itself, and thereby become inclusion defects.

[0005] Another type of exogenous inclusion, peculiar to titanium andother reactive metal alloys with solvus temperatures that rise withinterstitial oxygen, nitrogen or carbon content, is “hard alpha”, alsoknown as Type I inclusions. Hard alpha inclusions originate within suchalloys during process operations, such as welding, flame cutting,grinding, cutting and even furnace air leaks, that expose the moltenalloy to elements in air, particularly oxygen, nitrogen, and carbon.When such exposure occurs, the alloy takes the elements into solutionwhere the elements simultaneously stabilize and embrittle the alphaphase of the alloy. By stabilizing and embrittling the alpha phase, adefect is created within the alloy that is very similar in most otherrespects to the base alloy. Particulate debris from such operations caninadvertently migrate to the casting furnace and enter the ceramic shellmold as the casting pour takes place. Because hard alpha inclusions havea melting point exceeding that of the clean alloy, hard alpha inclusionscan survive exposure to the melt, enter the mold and become a brittleinclusion. Additionally, hard alpha inclusions can enter the primarymetal supply stream and unknowingly become part of the melt stock.

[0006] As stated, hard alpha inclusions originate during certain processoperations within titanium and other reactive metal alloys with solvustemperatures displaying a positive slope as oxygen, nitrogen or carbonare added. Such process operations are integral to other process streamsat the foundry where the components are manufactured and, as such, theprocess operations cannot be totally eliminated or isolated from thecasting activity. As a result, detailed contamination control plans aretypically implemented to prevent the generation and introduction of hardalpha debris into the foundry. Such contamination control plans,however, have a number of drawbacks. While contamination control plansaid in preventing the generation and introduction of hard alpha debris,such control plans generally do not remove hard alpha debris thatactually do form or become introduced from operations external to thefoundry. Also, contamination control plans typically add cost to themanufacture of the components, and add time required in the productionschedule of the components. Additionally, such contamination controlplans are generally difficult to enforce among manufacturers, and cannotbe easily validated. In this regard, because of the limitations ofcontamination control plans and the associated risk of componentfailure, the design of components made from such alloys still typicallyaccounts for the presence of hard alpha inclusions.

SUMMARY OF THE INVENTION

[0007] In light of the foregoing background, the present inventionprovides a system and filter for filtering hard alpha inclusions fromreactive metal alloys. In contrast to conventional contamination controlplans, the system and filter of the present invention remove hard alphadebris that forms or becomes introduced into the foundry. As such, thedesign of components made from relevant alloys manufactured according tothe present invention need not account for the presence of hard alphainclusions. Also, the system and filter add relatively little cost tothe manufacture of components, when compared to the cost to develop andimplement a conventional detailed contamination control plan. Utilizingthe system and filter of the present invention does not add anyappreciable time to the production schedule of components. Further,because the system and filter of the present invention remove hard alphainclusions as opposed to attempting to control or limit the formation ofsuch contaminants, no need exists to enforce or validate implementationof the filter as required by conventional contamination control plans.

[0008] According to one embodiment, the present invention provides asystem for filtering hard alpha inclusions from a reactive metal alloy,such as titanium. The system includes a vessel that is capable ofholding the reactive metal alloy in a molten form, and can pour themolten reactive metal alloy. The system also includes a receptacle forreceiving the molten reactive metal alloy poured from the vessel. And toprevent at least some hard alpha inclusions from entering thereceptacle, the system includes a filter disposed between the vessel andthe receptacle through which the molten reactive metal alloy passesbefore being received by the receptacle.

[0009] The filter includes a frame, and a porous surface that isdisposed within the frame such that the frame extends peripherally aboutthe porous surface. The porous surface defines a plurality of openingsthat are sized to permit the reactive metal alloy in molten form to passtherethrough. The filter is comprised of a material having a meltingpoint that exceeds a melting point of the reactive metal alloy and is atleast partially insoluble in the molten reactive metal alloy. Thematerial of the filter can have a solubility less than a predeterminedpercent by weight in the molten reactive metal alloy, such as less thantwenty-five percent by weight. Also, the material of the filter can havea melting point greater than a melting point of the reactive metal alloyby at least a predetermined amount, such as at least 500 degreesCelsius. For example, the filter can comprise an alloy including atleast one of niobium, molybdenum, tantalum, rhenium and tungsten.

[0010] Further, to limit solidifying of the molten reactive metal alloyon the filter as the molten reactive metal alloy passes through thefilter, the system can include a heating element in thermal contact withthe filter. More specifically, the heating element can preheat thefilter to thereby limit the solidifying of the molten reactive metalalloy within the openings defined by the porous surface of the filter.And in another embodiment, the system further includes a chamberdefining an internal cavity within which the vessel, receptacle andfilter are disposed. In this embodiment, the internal cavity is isolatedfrom an external environment. Also, the heating element is capable ofpreheating the filter by passing current through the filter.

[0011] The system and filter of the present invention, therefore, filterhard alpha inclusions from reactive metal alloys with solvustemperatures displaying a positive slope. In contrast to conventionalcontamination control plans, the system and filter of the presentinvention add relatively little cost to the manufacture of components,and do not add any appreciable time to the production schedule ofcomponents. Further, in contrast to conventional contamination controlplans, the system and filter of the present invention filter out hardalpha inclusions as opposed to attempting to control or limit theformation of such contaminants. And as such, no need exists to enforceor validate implementation of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Having thus described the invention in general terms, referencewill now be made to the accompanying drawings, which are not necessarilydrawn to scale, and wherein:

[0013]FIG. 1 is a block diagram of a system for filtering hard alphainclusions from a reactive metal alloy, according to one embodiment ofthe present invention;

[0014]FIG. 2 is a schematic front view of a filter according to oneembodiment of the present invention with an exploded inset of a portionof the filter;

[0015]FIG. 3 is a graph illustrating the solubility per weight at themelting temperature of several refractory metals in accordance with oneembodiment of the present invention; and

[0016]FIG. 4 is a schematic view of a vacuum arc skull melting andcasting furnace system including a filter in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0018] Referring to FIG. 1, a system 10 for filtering hard alphainclusions from reactive metal alloys includes a vessel 12, a receptacle14 and a filter 16. The reactive metal alloys can comprise any of anumber of alloys that are susceptible to hard alpha inclusions, such astitanium. In this regard, hard alpha inclusions originate within alloysduring process operations that expose the molten reactive metal alloy toelements in air, particularly oxygen, nitrogen, and carbon. When suchexposure occurs, the alloy takes the elements into solution where theelements simultaneously stabilize and embrittle the alpha phase of thealloy. In this regard, reactive metal alloys that are susceptible tohard alpha inclusions generally include those alloys with solvustemperatures that rise with interstitial oxygen, nitrogen or carboncontent. These reactive metal alloys include those metal alloys thatexperience an increase in melting point as the oxygen, nitrogen orcarbon content of the alloy increases, thus allowing local regions ofhigh oxygen, nitrogen or carbon content to persist during melting of thealloy. For example, the reactive metal alloy can comprise any alloyincluding elements such as zirconium, hafnium, and yttrium. But in apreferred embodiment, the reactive metal alloy comprises titanium.

[0019] The vessel 12 is capable of holding the reactive metal alloy inmolten form 18, and is capable of pouring the molten reactive metalalloy. The vessel can comprise any of a number of different materials solong as the vessel is capable of maintaining the vessel's mechanicalstrength and rigidity, and as long as the vessel does not react with orcontaminate the molten reactive metal alloy. To allow the vessel to holdthe molten reactive metal alloy without reacting with the metal alloy,the vessel can include an alloy “skull” layer of solidified reactivemetal alloy on the inside surface of the vessel between the vessel andthe molten reactive metal alloy, as such is known to those skilled inthe art. For example, when the reactive metal alloy comprises titanium,the vessel can comprise a copper crucible with a titanium skull layer onthe inside surface of the crucible between the molten titanium and thecrucible.

[0020] The receptacle 14 is capable of receiving the molten reactivemetal alloy poured from the vessel 12. The receptacle can comprise anyof a number of different materials but, similar to the vessel, thereceptacle is capable of maintaining the receptacle's mechanicalstrength and rigidity at high temperatures, and does not react with themolten reactive metal alloy 18. For example, the receptacle can comprisea ceramic mold for a component of a complex system, such as a fan for anaircraft engine. Also similar to the vessel, the receptacle can includea layer of material on the inner surface of the receptacle to allow thereceptacle receive the molten reactive metal alloy without reactingtherewith. For example, when the molten reactive metal alloy comprisestitanium, the innermost layer of the receptacle (that faces the moltenmetal) can include a layer of rare earth metal oxide(s), such as erbia,that has a high melting point and chemical compatibility with thereactive molten titanium.

[0021] As previously stated, particular molten reactive metal alloys candevelop hard alpha inclusions when the molten reactive metal alloy isexposed to elements in air, such as oxygen and nitrogen. As such, tofilter hard alpha inclusions from the molten reactive metal alloy 18poured from the vessel 12 into the receptacle 14, the system 10 includesthe filter 16. The filter is disposed between the vessel and thereceptacle and can be mounted in any manner therebetween. For example,the filter can be mounted directly over the opening of the vessel suchthat the molten reactive metal alloy passes through the filter as thevessel pours the molten reactive metal alloy into the receptacle.Alternatively, the filter can be mounted directly over the opening ofthe receptacle such that the molten reactive metal alloy poured from thevessel passes through the filter before the receptacle receives themolten reactive metal alloy.

[0022] As shown in FIG. 2, the filter 16 includes a frame 20, and aporous surface 22 disposed within the frame such that the frame extendsperipherally about the porous surface. The porous surface defines aplurality of openings 24 that allow the molten reactive metal alloy 18to pass through the porous surface. In one embodiment, the poroussurface comprises a plurality of metal strands, such as wires, thatextend across the frame in different directions and cross one another tothereby define the plurality of openings. As described below, theopenings have a square shape, but it should be understood that theopenings can be any shape without departing from the spirit and scope ofthe present invention.

[0023] The openings 24 of the filter 16 should be designed with asufficient area such that the molten reactive metal alloy 18 can passthrough the filter at a desired flow rate without accumulating on theporous surface 22 of the filter. But the area of the openings shouldalso be small enough that the porous surface maintains the filter'smechanical strength and rigidity as the molten reactive metal alloypasses therethrough. Additionally, the area of the openings should besmall enough so that solid contaminants, particularly hard alphainclusions, do not pass through the filter. In this regard, the area ofthe openings can be selected to thereby select the maximum size of thehard alpha inclusions allowed through the filter. For example, theopenings can be selected to have dimensions x and y, that equal oneanother and are equal to 0.070 inches. Also, the thickness, t, of theporous surface between the openings (or diameter of the wires if theporous surface comprises such) can be selected to equal 0.0050 inches.Thus, the porous surface would define a plurality of openings that are87% open (i.e., 0.070²/[0.070+0.0050]²). As another example, the poroussurface can define openings that are selected to be 90% open, with thethickness, t, selected to be between 0.005 and 0.010 inches, and the xand y dimensions determined accordingly.

[0024] Again referring to FIG. 1, to limit the solidification of themolten reactive metal alloy on the porous surface 22 of the filter 16and, particularly, within the openings 24, the system 10 can furtherinclude a heating element 26. The heating element is capable ofpreheating the filter prior to and/or during pouring of the moltenreactive metal alloy from the vessel 12. The heating element cancomprise any of a number of different devices as such are known. Theheating element is capable of preheating the filter to a temperaturesufficient to limit the amount of molten reactive metal alloy thatsolidifies on the filter to as little as possible. In one embodiment,the system further includes a chamber 28, such as a vacuum chamber, thatdefines an internal cavity within which the vessel, receptacle 14 andfilter are disposed. In this embodiment, the heating element can preheatthe filter by passing current through the filter sufficient to preheatthe filter to a desired temperature.

[0025] Referring now to FIG. 3, the porous surface 22 of the filter 16can comprise any of a number of different refractory metals orrefractory metal alloys. Similarly, the frame 20 can comprise any of anumber of different refractory metals or refractory metal alloys. In apreferred embodiment, the frame comprises the same material as theporous surface, although the frame and porous surface can comprisedifferent materials without departing from the spirit and scope of thepresent invention. The refractory metal or refractory metal alloy isselected such that the refractory metal or refractory metal alloy has amelting point that exceeds a melting point of the reactive metal alloyand is at least partially insoluble in the molten reactive metal alloy18. In this regard, the refractory metal or refractory metal alloy maybe selected to have a solubility less than a predetermined percent byweight in the molten reactive metal alloy, and a melting point greaterthan the melting point of the reactive metal alloy by at least apredetermined amount. By so selecting the refractory metal or refractorymetal alloy, the material of the filter has the least chance ofcontaminating the molten reactive metal alloy as the molten reactivemetal alloy passes therethrough, and the material is the leastsusceptible to creep or deflection.

[0026] Therefore, with respect to titanium, the materials located in thelower right region of the graph of FIG. 3 include those refractorymetals that have a sufficiently low solubility in molten titanium at asufficiently high melting temperature from which the porous surface 22and/or the frame 20 of the filter 16 can be made. As is known, titaniumhas a melting point of approximately 1660 degrees Celsius (designated inFIG. 3 by the broken line). As an example, the material can be selectedto have a melting point at least 500 degrees Celsius more than themelting point of titanium. Also, the material may be selected to have asolubility less than twenty-five percent by weight at the melting pointof the material. It should also be understood, however, that thematerial of the porous surface and/or the frame can be selected withother differences in melting temperature and other amounts ofinsolubility if so desired. As shown, for example, in the preferredembodiment where the molten reactive metal alloy 18 comprises moltentitanium, the porous surface comprises tungsten, W. It should beunderstood, however, that the porous surface and/or the frame can alsocomprise niobium, Nb, tantalum, Ta, molybdenum, Mo, and/or rhenium, Re.Alternatively, the porous surface and/or the frame can comprise arefractory metal alloy that includes at least one refractory metalselected from W, Nb, Ta, Mo and Re, as well as vanadium, V, rhodium, Rhand hafnium Hf.

[0027] Referring to FIG. 4, one type of system that would benefit fromthe filtering of the present invention is depicted. As shown, a vacuumarc skull melting and casting furnace 30 generally includes avacuum-tight chamber 32 in which molten metal alloy 34 is driven downinto a crucible 36 (i.e., vessel). A power supply 38 provides current tostrike an electric arc between a consumable electrode 40 and thecrucible to thereby melt the reactive metal alloy in the crucible. Thecrucible can be water cooled and, as such, a solidified metal alloyskull can form on the inner surface of the crucible thereby shieldingthe crucible from direct contact with the molten metal alloy. Once aspecified amount of molten metal alloy 34 is contained within thecrucible 36, the electrode 40 can be retracted. Thereafter, the crucibleis tilted to pour the molten metal alloy into an investment casting mold42 (i.e., receptacle) positioned beneath the crucible. Before the moltenmetal alloy reaches the investment casting mold, however, the moltenmetal alloy can pass through a filter 44 designed in accordance with thepresent invention. The filter is disposed between the crucible and theinvestment casting mold. And by passing the molten metal alloy throughthe filter, solid contaminants, particularly hard alpha inclusions, canbe removed from the molten metal alloy.

[0028] It will be appreciated that the system and filter of the presentinvention can be utilized in many different applications, of which thevacuum arc skull melting and casting furnace used in investment castingis one example. The system and filter of the present invention wouldbenefit any system or process where molten reactive metal alloy that issusceptible to hard alpha inclusions or other contaminants. For example,the system and filter of the present invention can be used to purifymolten reactive metal alloy by removing hard alpha inclusions and othercontaminants from molten reactive metal alloy supply stock for forging,extrusions or the like.

[0029] Therefore, the present invention provides a system and filterthat adds relatively little cost to the manufacture of components, whencompared to the cost to develop and implement a conventional detailedcontamination control plan. Also, no need exists to enforce or validateimplementation of the system and filter of the present invention, incontrast to conventional contamination control plans. In this regard,the design of components made from relevant alloys manufacturedaccording to the present invention need not account for the presence ofhard alpha inclusions larger than the size of the openings of thefilter. Systems utilizing such improved components will benefit fromimproved reliability and safety, and reduced cost, due to extended safeoperating life and reduced inspection.

[0030] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation

What is claimed is:
 1. A system for filtering hard alpha inclusions froma reactive metal alloy, said system comprising: a vessel capable ofholding the reactive metal alloy in a molten form, wherein said vesselis capable of pouring the molten reactive metal alloy; a receptacle forreceiving the molten reactive metal alloy poured from said vessel; and afilter disposed between said vessel and said receptacle through whichthe molten reactive metal alloy passes before being received by saidreceptacle for preventing at least some hard alpha inclusions fromentering said receptacle, said filter comprised of a material having amelting point that exceeds a melting point of the reactive metal alloyand is at least partially insoluble in the molten reactive metal alloy.2. A system according to claim 1 further comprising: a heating elementin thermal contact with said filter, wherein said heating element iscapable of preheating said filter so as to limit solidifying of themolten reactive metal alloy on said filter as the molten reactive metalalloy passes therethrough.
 3. A system according to claim 2 furthercomprising: a chamber defining an internal cavity within which saidvessel, receptacle and filter are disposed, wherein the internal cavityis isolated from an external environment, and wherein said heatingelement is capable of preheating said filter by passing current throughsaid filter.
 4. A system according to claim 2, wherein said filtercomprises a porous surface defining a plurality of openings, and whereinsaid heating element preheats said filter to thereby limit thesolidifying of the molten reactive metal alloy within the openingsdefined by said porous surface of said filter.
 5. A system according toclaim 1, wherein at least a portion of said filter comprises arefractory metal alloy including at least one of niobium, molybdenum,tantalum, rhenium and tungsten.
 6. A system according to claim 1,wherein said filter includes a porous surface defining a plurality ofopenings, and wherein the porous surface comprises a refractory metal.7. A system according to claim 6, wherein the refractory metal isselected from a group consisting of niobium, molybdenum, tantalum,rhenium and tungsten.
 8. A system according to claim 1, wherein thereactive metal alloy with a solvus temperature displaying a positiveslope comprises titanium.
 9. A system according to claim 1, wherein saidfilter comprises: a frame; and a porous surface disposed within saidframe such that said frame extends peripherally about said poroussurface, wherein said porous surface defines a plurality of openingsthat are sized to permit the reactive metal alloy in molten form to passtherethrough.
 10. A system according to claim 1, wherein said filtercomprises a material having a solubility less than a predeterminedpercent by weight in the molten reactive metal alloy.
 11. A systemaccording to claim 10, wherein the material of said filter has asolubility less than twenty-five percent by weight in the moltenreactive metal alloy.
 12. A system according to claim 1, wherein saidfilter comprises a material having a melting point greater than amelting point of the reactive metal alloy by at least a predeterminedamount.
 13. A system according to claim 12, wherein the material of saidfilter has a melting point greater than a melting point of the reactivemetal alloy by at least 500 degrees Celsius.
 14. A filter for filteringhard alpha inclusions from a reactive metal alloy, said filtercomprising: a frame; and a porous surface disposed within said framesuch that said frame extends peripherally about said porous surface,wherein said porous surface defines a plurality of openings that aresized to permit the reactive metal alloy in molten form to passtherethrough while separating at least some hard alpha inclusionstherefrom, said porous surface comprised of a material having a meltingpoint that exceeds a melting point of the reactive metal alloy and is atleast partially insoluble in the molten reactive metal alloy.
 15. Afilter according to claim 14, wherein said filter is formed of athermally conductive material that is capable of being preheated so asto limit solidifying of the molten reactive metal alloy on the filter asthe molten reactive metal alloy passes through said porous surface. 16.A filter according to claim 14, wherein said porous surface comprises arefractory metal alloy including at least one of niobium, molybdenum,tantalum, rhenium and tungsten.
 17. A filter according to claim 14,wherein said porous surface comprises a refractory metal.
 18. A filteraccording to claim 17, wherein the refractory metal is selected from agroup consisting of niobium, molybdenum, tantalum, rhenium and tungsten.19. A filter according to claim 14, wherein said porous surfacecomprises a material having a solubility less than a predeterminedpercent by weight in the molten reactive metal alloy.
 20. A filteraccording to claim 19, wherein the material of said porous surface has asolubility less than twenty-five percent by weight in the moltenreactive metal alloy.
 21. A filter according to claim 14, wherein saidporous surface comprises a material having a melting point greater thana melting point of the reactive metal alloy by at least a predeterminedamount.
 22. A filter according to claim 21, wherein the material of saidporous surface has a melting point greater than a melting point of thereactive metal alloy by at least 500 degrees Celsius.